The present invention relates to a control device of a multi-cylinder engine.
Conventionally, a cylinder-number controlled engine having a plurality of cylinders is well known. This type of engine switches its operation mode between an all-cylinder operation in which all the cylinders of the engine are activated and a reduced-cylinder operation in which the operation of specific cylinder(s) of the plurality of cylinders is suspended (e.g., see JP2004-270491A). JP2004-270491A discloses an engine control device for construction machineries. The engine control device includes a variable displacement oil pump which is operated by a cylinder-number controlled engine, and in the reduced-cylinder operation, the engine control device controls the driven cylinder(s) to reduce an engine speed down to an idle speed and minimizes a discharge flow rate of the oil pump, so as to save the fuel.
Moreover, for example, JP1996-165948A discloses a speed control device of an engine which feedback-controls an idle speed with an ISC valve (idle speed control valve). The speed control device is provided with an air amount control valve for auxiliary component load which controls a discharge pressure of a pump for auxiliary component load to change an intake air amount. The speed control device suppresses a variation of the idle speed due to an auxiliary component load variation, by estimating an air amount passing through the air amount control valve for auxiliary component load, and controlling an operation amount of the ISC valve according to a change amount of the passing air amount per unit time.
Meanwhile, a case can be considered where a valve stopping device having a locking mechanism for stopping the operation of at least one of an intake valve and an exhaust valve is provided to each specific cylinder which is suspended in the reduced-cylinder operation, and the operation of the at least one of the intake and exhaust valves of the specific cylinder is stopped by using the locking mechanism of the valve stopping device in the reduced-cylinder operation. In this case, a configuration can be considered for the locking mechanism, for example, which switches a state of the valve between operated and non-operated by switching a state of components configuring a valve operating mechanism between coupled and uncoupled.
However, when such a locking mechanism is provided to the valve operating mechanism, a clearance occurs in the locking mechanism, and this clearance varies depending on a manufacturing error. Considering the clearance, a valve lift in a period of a ramp part of a cam lift property of each valve of the specific cylinder needs to be larger than that of the cylinder other than the specific cylinder.
For this reason, there is a case where a variable valve timing mechanism (VVT) is used to cause open periods of the intake and exhaust valves to overlap and change the overlapping amount (valve overlapping amount) according to an operating state of the engine. In this case, since there is a limit in changing the valve overlapping amount with the VVT, in order to increase the valve overlapping amount in a high engine load state as much as possible, it is desirable to cause the open periods of the intake and exhaust valves to overlap even in a low engine load state (e.g., in idling). The valve overlapping amount in this low engine load state is determined in consideration of combustion stability. Specifically, when the valve overlapping amount is increased, an internal EGR (exhaust gas recirculation) amount becomes relatively large compared to a fresh air amount in each cylinder, and thus, the combustion stability degrades. Therefore, it is preferred that the valve overlapping amount in the low engine load state is set to a largest value possible within the extent that the combustion stability can be secured.
Further, when the locking mechanism is provided to the valve operating mechanism as described above, since the valve lift in the period of the ramp part increases in the specific cylinder, the valve overlapping amount of the specific cylinder in the low engine load state becomes larger than the valve overlapping amount of the cylinder other than the specific cylinder in the low engine load state. Therefore, even if the valve overlapping amount of the cylinder other than the specific cylinder in the low engine load state is set to the largest value possible within the extent that the combustion stability can be secured as described above, the valve overlapping amount of the specific cylinder in the low engine load state becomes larger than the largest value, and thus, the combustion stability in the specific cylinder degrades, and as a result, there is a possibility that the engine torque varies.
In the low engine load state (e.g., in idling), it is preferred to perform the all-cylinder operation since the engine torque variation becomes even larger if the reduced-cylinder operation is performed as described in JP2004-270491A; however, even in the all-cylinder operation, in the case where the locking mechanism is provided to the valve operating mechanism as described above, the possibility that the engine torque varies becomes higher due to the degradation of the combustion stability in the specific cylinder. Moreover, when the discharge flow rate of the oil pump is minimized in the low engine load state (e.g., in idling) as in JP2004-270491A, since the engine load is reduced, the intake air amount is reduced, the internal EGR amount with respect to the fresh air amount is increased even more in each cylinder, and the combustion stability particularly in the specific cylinder degrades more, resulting in increasing the engine vibration.
On the other hand, it can be considered to reduce the valve overlapping amount for all the cylinders by changing either one of an open timing of the intake valve (IVO) and a close timing of the exhaust valve (EVC). However, when the IVO or the EVC is changed, the engine performance (e.g., the fuel consumption and the output) degrades outside the low engine load range.
Further, even if a configuration in which the open periods of the intake and exhaust valves do not overlap in the low engine load state is adopted, by providing the locking mechanism to the specific cylinder only, especially when a total operating time length of the engine becomes long, valve open-close properties easily become different between the specific cylinder and the other cylinders. Thus, there is a possibility that the engine torque varies. JP2004-270491A and JP1998-165948A do not disclose any countermeasure for this problem.